Like many species that live in perpetual darkness, the blind cavefish has lost its eyes. These fish have evolved from sighted ancestors on several occasions in different Mexican caves. Their eyes have degenerated over a million years of darkness, but their blindness can be easily reversed by a spot of clever breeding. Many genes govern the development of eyes, and different populations of cavefish have lost their vision by disrupting different eye genes. By breeding individuals from different caves, working genes from one parent can compensate for broken ones from another. The result: babies that can see. (Photo by skpy)